Electrically heated film evaporator



United States Patent 3,247,888 ELECTRICALLY HEATED FILM EVAPORATORHeinrich Mueller, Ludwigshafen (Rhine), and Werner Honsberg, BadDuerkheim, Germany, assignors to Badische Anilin- & Soda-FabrikAktiengesellschaft, Ludwigshafen am Rhine, Germany Filed June 25, 1963,Ser. No. 290,458 Claims priority, application Gtirmany, June 27,

7,83 Claims. (Cl. 159-13) This invention relates to an electricallyheated evaporator. More specifically it relates to a film evaporator forvaporizing caustic alkali solutions.

It is known to dehydrate or concentrate caustic alkali solutions bymeans of film evaporators. The use of such apparatus offers severaladvantages. Owing to the continuous operation, operating costs are low.Moreover, the thin liquid film to be dehydrated does not have to beheated too much so that stress on the material is slight. The goodthermal efiiciency of film evaporators is another notable feature.

Film evaporators of the type mentioned may consist for example of avertical or substantially vertical tube preferably made of nickel. Thesolution to be dehydrated flows down the inner surface of the tube as acoherent film. If at the same time-the tube is heated, and the amount ofheat supplied and the solution fed in bear a certain relation to eachother, it may be expected that the water will be vaporized for the mostpart and that a substantially anhydrous melt will be discharged at thelower end of the tube.

If the said relationship is changed, then either only a portion of thewater evaporates, so that the solution of caustic soda is for exampleconcentrated from 50% to 70%, or an anhydrous melt may be obtained whichmay be superheated.

While the first variant of the process may be used industrially,overheating of the melt is avoided so that the regulation of the heatsupplied and of the amount of caustic alkali solution fed in. It hasbeen found to be advantageous to heat the tube of a film evaporatorelectrically. One prior art heating means comprises electrical heatingelements arranged concentrically along the length of the evaporatortube.

This arrangement however has great disadvantages. The heat istransmitted from the heating elements via a heat-resistant protectivelayer to an inner tube of a nickelchromium-cobalt alloy which in turnheats the evaporator tube by radiation. Owing to the high heatresistance between heating element, protective layer, inner tube andevaporator tube, it is only after about four hours that thermalequilibrium is set up, because it is only after this period that theheating element has achieved its end temperature. Moreover the specificloading of the heating surface of the evaporator tube is low, a tubehaving an external diameter of 125 mm. and a length of 3.96 m. handlingonly 72 kg./h. of 50% caustic alkali solution. An evaporation capacityof 19 kg./h. of water per square meter of surface of the evaporator tubecan be calculated from these values. Another disadvantage of thisarrangement is that it is impossible to shut off the plant within ashort time because the high temperature of the heating 3,247,888Patented Apr. 26, 1966 trically heated film evaporator whose heating-upcan be quickly and easily controlled. Another object of the invention isto provide a film evaporator which can be heated within' a short period.Yet another object of the invention is to produce different amounts ofheat along the length of the evaporator tube according to the heatrequired in the tube.

These and other objects and advantages of the invention are achieved bythe electrically heated film evaporator according to this inventionhaving a preferably vertically arranged evaporator tube. It ischaracterized by the following features: (a) the evaporator tube isarranged as an electrical resistor in the circuit; (b) a feed line isprovided at or near the upper end of the tube for the supply of thematerial to be dehydrated to the inner wall of the evaporator tube; and,(c) the evaporator tube is provided at or near each end with a contactplate for the supply of electric current.

Beneath the feed line for the material to be dehydrated, the evaporatortube may be tapped for uniform distribution of the liquid fed in.

Over the length between the two contact plates, it is advantageous tomake the wall of the evaporator tube of different thicknesses accordingto the heat required.

In order that the usual electric voltages may be used without loss, aplurality of evaporator tubes may be provided, preferably connected inseries.

The use of the evaporator tube as an electric conductor generating theheat within the wall of the tube is the most favorable solution becausethe tube has alternating current flowing either through its whole lengthor through only a portion thereof so that the tube is heated accordingto requirements. The heat may be varied within wide limits by-regulationof the current and voltage. The evaporator tube may be enveloped insuitable insulating material, for example mineral wool, to keep heatloss low.

The caustic alkali solution is supplied at or near the top of theevaporator tube, flows down on the inner wall of the tube and leaves atthe bottom of the tube dehydrated to a greater or lesser degree,depending on the temperature -to which it has been exposed. Then it iscollected. The evaporated water flows in the opposite direction, passesthrough the upper end of the tube as vapor mixed with a small amount ofair, and is passed to a condenser which condenses the water vapor. Theremaining air is blown into the atmosphere by the action of a fan. Theoutput of the fan is controlled so that the amount of air sucked in isjust suflicient to prevent water vapor issuing from the lower end of theevaporator tube.

We have found that when working in this way, in contrast to the priorart methods, no expensive distribution of the caustic alkali solution onthe evaporator tube is necessary. It is suflicient if the solution issupplied at or near the upper end of the evaporator tube through a feedline which is mounted radially or tangentially on the evaporator tube.The solution is immediately distributed uni formly on the inner surfaceof the tube and flows down. The only measure recommended is thattheinner wall of the tube should be roughened or tapped for a shortdistance, for example 50 to mm., below the point at which the liquidenters.

Using the evaporator tube as an electric conductor offers the additionaladvantage that the metal cross-section of the tube may be varied at willby metals welded onto the tube or by the use of tubes having differentwall thickness or unequal diameter. This makes it possible to vary theheat supplied to different parts of the tube. For

example it is advantageous if the lower end of the heated portion of thetube, for about 50 cm., has twice the metal cross-section of theremainder of the length of the tube and consequently, as compared withthe rest of the tube, receives only one quarter of Joules energy. Inthis way it is possible easily to maintain any desired temperature ofthe effluent melt and to avoid overheating.

The use of the evaporator tube as an electric conductor otters greatadvantages in operation. The heating-up period when starting up fromcold is only a few minutes. During operation, the temperature of thetube is not very much higher than the temperature of the solution.Therefore the risk of overheating is very slight. The

.electric current and the liquid feed may be shut off at the same timewithout the risk of the evaporator tube becoming overheated andtherefore strongly corroded. Furthermore, in this apparatus it is notnecessary to produce, at the beginning and end of the evaporation, amelt which is only partly dehydrated.

The invention will now be described with reference to the accompanyingdrawing which illustrates diagrammatically by way of example a series ofthree electrically heated evaporator tubes according to the invention.The actual dimensions used will vary according to the evaporationcapacity.

Three vertical nickel' tubes 1a, 1b and 1c are connected electrically inseries by means of the contact plates 2, 3, 2 and 3 which are in elasticconnection with each nickel tube, and an alternating current is suppliedto the electrically connected tubes. Each nickel tube is 7 In. in lengthand 70 mm. in internal diameter and has a wall thickness of 2.5 mm. inthe upper portion and an increased wall thickness of mm. in the bottomportion extending about 50 cm. axially upwardly from the bottom of theheated portion of the tube. Each tube as shown thus serves as anelectrical conductor for about 6 m. of its length.

Since all three tubes are substantially identical in structure andoperation, the following description with reference to one of thesetubes l ra-applies also to the remaining tubes 1b and 1c except asotherwise noted. Reference numerals are the same for the structure ineach tube.

Caustic soda solution is introduced through a feed line 4a to the upperend of the nickel tube 1a and above the contact plate 2 (plate 2' intubes 1b and 1c), is distributed along a tapped portion 5a of the innerwall 6a of the tube, flows down the inside of the tube as a uniform filmand leaves the tube as a dehydrated melt at the lower end 7a thereof andcan be collected there. The roughened portion 5b of the inner wall 6b oftube 1b is likewise shown as a tapped thread, while this roughenedportion 50 on the inner wall 6c of tube is in the form of across-notched area. In each case the roughened portion is about 100 mm.long in the axial direction and is located immediately downstream fromthe feed line.

Water vapor and sucked-in air are drawn away by fan through the upperends 8a, 8b and 8c of the tubes and passed through the manifold 10 and acondenser 11 in which the heat exchange fluid is introduced at 12 andremoved at 13. The condensed liquid is withdrawn, through line 14 andgases are ventilated by the fan 15. Each evaporator tube is heatinsulated with mineral wool 9a, 9b and 90.

If the tube 1a is loaded with a current of 5000 amp, a voltage drop ofabout 12 volts is produced in the heated condition. At the same timeabout 100 kg./ h. of a 50% caustic soda solution is metered in throughthe feed line 4a. The solution is dehydrated in contact with the innerwall do. When the melt leaves the tube at 7a, it still contains about0.5 to 1% of water. Its temperature is 380 to 400 C.

Each square meter of the inner surface of the tube 10 thereforeevaporates 38 kg. of water per hour. This value is not however themaximum value. Loading of the tube wall with electric current is limitedby the velocity of the vapor at the upper end and this shouldadvantageously not exceed 10 m./sec.

Each of the tubes 1b and 1c can be operated in the same manner under thesame conditions so as to handle a total feed to all three tubes of 300kg./h. of the 50% caustic soda solution. While only three tubes areillustrated in the drawing, it is advantageous in an industrial plant tobank together six or more tubes of appropriate length since this -willgive a satisfactory voltage drop when connected electrically in series.

The evaporator described is also suitable for dehydrating caustic alkalisolutions having higher contents of caustic alkali, for example 71%caustic soda solution, which is to be regarded as a melt because itsolidifies at room temperature. It is advantageous to heat up thecaustic alkali solutions by cheaper sources of energy than electricalenergy to a temperature at or near the boiling points prior toevaporation and only then to supply the solutions to the filmevaporator. For example 50% caustic soda solution may with advantagefirst be concentrated to 71% by means of conventional mul-tipassevaporators and then dehydrated in the electrically heated filmevaporator.

We claim:

1. An electrically heated film evaporator for dehydrating caustic alkalisolutions comprising: a substantially vertical evaporator tube, saidevaporator tube being connected in an electrical circuit as anelectrical resistance; means to uniformly distribute the solution to bedehydrated on the inner wall surface of said tube including (A) a feedline for supplying the solution at an angle to the axis of said tube toimpinge on the inner wall of said tube in the neighborhood of the upperend of said tube and (B) a roughened portion on the inner wall of saidtube extending over a short axial distance within said tube beneath andimmediately downstream from said feed line; and means for supplyingelectric current to said evaporator tube.

2. An evaporator as claimed in claim 1 wherein the thickness of the wallof said evaporator tube varies inversely with local heat requirements.

3. An evaporator as claimed in claim 1 comprising a plurality of saidevaporator tubes connected electrically in series.

4. An evaporator as claimed in claim 1 wherein a fan is connected ingaseous communication with the upper end of said evaporator tube forwithdrawal of water vapor and air through said upper end.

5. An evaporator as claimed in claim 1 wherein said roughened portion onthe inner wall of said tube comprises a tapped thread extending up toabout mm. below said feed line.

References Cited by the Examiner UNITED STATES PATENTS 568,615 9/1896Haubtman 15913 912,994 2/1909 Conrad 219-400 1,727,585 9/1929 Carleton219-300 1,932,406 10/1933 Harris 159---13 2,873,799 2/1959 Earley et al.15913 2,891,375 6/1959 Vandamme et al. 5734 FOREIGN PATENTS 123,5122/1947 Australia.

891,660 12/ 1943 France.

482,715 4/1938 Great Britain.

NORMAN YUDKOFF, Primary Examiner. ANTHONY BARTIS, Examiner.

1. AN ELECTRICALLY HEATED FILM EVAPORATOR FOR DEHYDRATING CAUSTIC ALKALISOLUTIONS COMPRISING: A SUBSTANTIALLY VERTICAL EVAPORATOR TUBE, SAIDEVAPORATOR TUBE BEING CONNECTED IN AN ELECTRICAL CIRCUIT AS ANELECTRICAL RESISTANCE; MEANS TO UNIFORMLY DISTRIBUTE THE SOLUTION TO BEDEHYDRATED ON THE INNER WALL SURFACE OF SAID TUBE INCLUDING (A) A FEEDLINE FOR SUPPLYING THE SOLUTION AT AN ANGLE TO THE AXIS OF SAID TUBE TOIMPINGE ON THE INNER WALL OF SAID TUBE IN THE NEIGHBORHOOD OF THE UPPEREND OF SAID TUBE AND (B) A ROUGHENED PORTION ON THE INNER WALL OF SAIDTUBE EXTENDING OVER A SHORT AXIAL DISTANCE WITHIN SAID TUBE BENEATH ANDIMMEDIATELY DOWNSTREAM FROM SAID FEED LINE; AND MEANS FOR SUPPLYINGELECTRIC CURRENT TO SAID EVAPORATOR TUBE.